The present invention relates to a circuit device for electron multipliers, especially for photomultipliers, which allows the measurement of high light intensities with high linearity, short risetimes and optimal signal-to-noise ratios.
The signal-to-noise ratio for the output of a photomultiplier is given by the equation EQU S/N = .sqroot.I.sub.k /2.alpha.e.DELTA.f = .sqroot.2.tau..sub.D I.sub.k /.alpha.e (1)
where:
I.sub.k = cathode current in [A], PA1 e = 1.6 .sup.. 10.sup.-.sup.19 [A sec] PA1 .alpha. = power noise factor PA1 .DELTA.f = power bandwidth PA1 .tau..sup.D = detector risetime constant, referred to as "risetime"
Because of the short signal risetimes .tau..sub.D involved, processes which take place in the .mu.sec and nsec range can only be investigated with satisfactory signal-to-noise ratios if high cathode currents are used. This is especially true when an average value cannot be obtained from a large number of individual measurements. A typical example of such an application is the optical investigation of fast chemical reactions by a method such as temperature-jump relaxation. In this case the signal-to-noise ratio for a risetime of, say 0.3 .mu.sec, should be of the order of 10.sup.4, corresponding to cathode currents of up to 100 .mu.A or more. A very high linearity and a frequency response which is constant almost up to the cut-off frequency are required. The transient response must be characterized by fast settling and must be free from any instability and drifting effects in the long time range.
High cathode currents with good linearity can be obtained by using semiconductor and vacuum photodiodes. However, difficulties arise from the fact that one often has to work at much lower light intensities, too. In the case of monochromatic measurements in the visible and ultraviolet spectral ranges the maximal possible cathode currents can vary by a factor of 1000 or more. The use of photodiodes would then require either very high load resistances, leading to a long signal risetime .tau..sub.D, or, if small load resistances are used, large electronic postamplification. In the latter case the thermal noise of the load resistance, the amplifier noise, and the drifting or the amplifier would cause an inadmissible deterioration of the measuring signal.
Conventional photomultiplier circuits in which the cathode current is amplified by a large number of dynode stages are also unsuitable for the purpose mentioned. In continuous operation, only currents of the order of 100 .mu.A are permissible at the anode. At an amplification of 10.sup.5 this would correspond to a cathode current of only 1 nA. If, however, the amplification is reduced by decreasing the dynode voltage, linearity and noise performance will deteriorate, whereas excessively high currents will lead to drifting and fatigue.
A known technique for reducing the amplification of a photomultiplier is to decrease the number of active dynodes by connecting several dynodes in parallel with the anode. As a consequence, the range in which the amplification can be controlled by changing the dynode voltage is considerably limited. Changing the amplification by other means leads to the same difficulties as encountered with photodiodes.
Furthermore, there are circuits known by which the number of active dynodes can be changed allowing the use of a constant load resistance. For the electronic postamplification only a gain interval corresponding to the gain factor per stage v of the photomultiplier has to be covered (e.g. v = 4 . . . 5). The switching of the dynodes is usually done by purely mechanical switching of the photomultiplier connections with the dynode resistor chain and the signal output. This, however, requires a large number of coupled switches if a large number of dynodes is to be switched and becomes quite cumbersome. At the same time the stray capacitances and, consequently, the signal risetime .tau..sub.D will increase.
Another known circuit device uses switching diodes which are series-connected with the dynode chain resistors. In principal, only two coupled switches are needed in this circuit; one for selecting one dynode as the effective anode, and another one for connecting the corresponding point of the dynode resistor chain to ground. The switching diodes disconnect the effective anode and all higher dynodes from the dynode resistor chain. However, the diodes must have a good small-signal behaviour, especially low capacitances. Thus they may be easily destroyed when switching the high dynode potentials. At the same time, the potential of the unused higher dynodes becomes freely variable, leading to interference with the active part of the phototube and deteriorating linearity and frequency response. Furthermore, the circuit involves serious difficulties in paralleling capacitors to the dynode resistor chain in order to improve the high frequency response. Without such capacitors a very high current drain is needed for short risetimes.